1. Field of the Invention
This invention relates to compression refrigeration systems and, more particularly, to a refrigeration system for producing an extremely wide span of ultra-low and cryogenic temperatures while maintaining reasonable efficiency with an auto refrigerating cascade (ARC) system.
2. Brief Description of the Prior Art
The prior art is exemplified by the patent of Missimer U.S. Pat. Nos. 3,698,202 and 3,768,273. Missimer U.S. Pat. No. 3,768,273 provides a rather complete explanation of the operation of ARC refrigeration systems.
Missimer U.S. Pat. No. 3,698,202 describes a method of bypassing the final throttling device of a full-separation (FS) ARC system for purposes of facilitating start-up of such a system. This method, while effective with FS-ARC systems of two or less cascades, does not allow starting of systems with three or more cascades. This method does not lend itself to very wide temperature span applications. A specific embodiment would be limited to a practical maximum operating span of about 40 degrees C.
Missimer U.S. Pat. No. 3,768,273 describes a system which allows ARC systems of more than two cascades to start. However, the invention does not employ FS. Rather, it relies on partial separation (PS) with condensate carry over to facilitate start-up. The system, while effective, is limited to a practical maximum operating span of about 40 degrees C. and is not particularly fast to start. It is not as efficient as FS-ARC and therefore does not develop as much cooling capacity at any given temperature or get as cold at a given capacity.
A typical PS-ARC system designed to operate down to -140 degrees C. with 100 watts capacity can be operated up to a maximum temperature of -100 degrees C. with 1000 watts capacity. Higher temperature and capacity operation results in excessive operating pressure and temperature at the compressor, risking damage thereto. Warmer temperature operation can be achieved by attenuating the flow of the throttling device which feeds the final evaporator. The method also results in a subtle but serious problem. As the flow of refrigerant is decreased, the average temperature of the final evaporator increases, but the unit supplies colder refrigerant. This colder inlet and higher average temperature results in a large temperature gradient through the evaporator, an unacceptable situation for many applications.
A phenomenon occurs with FS-ARC and, to a lesser extent, with PS-ARC systems known as self-refrigeration. Self-refrigeration occurs when cascades in the middle of the heat exchanger chain become too cold. The refrigerant leaving the over-cooled cascade is mostly condensed and very little refrigerant continues through the phase separator vapor branch to the next cascade. Hold up of liquid in the heat exchangers also contributes to self-refrigeration. The result is that the final throttling device feeds a much reduced quantity of refrigerant to the evaporator. The cooling capacity of the system then falls to almost nothing and may not recover. In milder cases, typically in PS-ARC systems, the cooling capacity is reduced for several minutes until the unit automatically recovers. In both cases, the evaporator temperature rises during the period of reduced cooling capacity.
Self-refrigeration is triggered by quick changes in operating conditions, for example, start-up, rapid defrost or cooling of the evaporator (See Forrest U.S. Pat. No. 4,597,167) or sudden changes in heat load on the evaporator. Self-refrigeration during start-up is the reason simple FS systems do not start. It has been found that self-refrigeration manifests more as the number of cascades is increased. Self-refrigeration is seldon seen with two cascade systems, but affects all three cascade systems to some degree, and is anticipated to be severe with four or more cascade systems.